The GPS system uses a constellation of satellites which rotate about the earth in very precisely determined orbits, that is to say the position of an arbitrary satellite can be ascertained at any instant. The orbits of the satellites are chosen in such a way that at any time, 6 to 12 satellites are visible at any point of the earth. Each satellite transmits two radioelectric signals of frequencies L1 (1575.42 MHz) and L2 (1227.6 MHz). On the ground or on a vehicle on land or sea or in the air, a GPS receiver receives the signals transmitted by visible satellites.
The onboard GPS receiver measures the duration of propagation required in order for a time mark transmitted by a satellite to reach it. The time marks are coded on carrier waves by the phase modulation technique. Each satellite thus transmits its own pseudo-random code. A replica of the sequence of the code is generated by the receiver and the shift that the replica must undergo so as to coincide with the code received corresponds to the duration of propagation of the signal in order to travel the satellite-receiver distance. This duration multiplied by the speed of light in the medium traversed gives a distance measurement called a pseudo-distance. On the basis of the measurements of the pseudo-distances separating it from each visible satellite, and of the knowledge of the position of the satellites, the receiver deduces its precise position in latitude, longitude, and in altitude in a terrestrial reference frame by a numerical resolution procedure akin to triangulation. It can also deduce therefrom the date and the precise time in the temporal reference frame of the GPS system.
The time reference of the receiver, provided by its clock, does not coincide perfectly with the time reference of the satellites of the constellation; this induces a bias in the measurements of propagation time, therefore of distance, that is equal to the delay of the time reference of the receiver with respect to the time reference of the satellites. The term “pseudo-distance” is employed for this purpose. The time bias, common to all the measurements, constitutes a fourth unknown, in addition to the three position unknowns, and this makes it necessary to have at least four measurements to calculate the position.
Furthermore, the position of the receiver is estimated by making a certain number of approximations. The measurement of the pseudo-distance cannot for example circumvent the system-related errors such as the lack of precision of the ephemerides or clocks onboard the satellites. The measurement of the pseudo-distance is also marred by errors related to the interactions between the signals and the atmospheric layers that they pass through. The signal propagation delay in the troposphere and the ionosphere depends on the inclination of the path and the time at which it takes place. Typically, GPS positioning errors related to the atmosphere are more marked by day than at night and more sensitive when a satellite is close to the horizon than the zenith. In certain applications such as for example a precision approach in aeronautics, the positioning precision obtained by a direct (or absolute) measurement of the pseudo-distance is not sufficient.
The use of a differential measurement makes it possible to substantially improve the precision of the positioning. It consists in transmitting, via a dedicated channel (VHF, UHF or cellular telephony), corrections of the pseudo-distance measurements formulated on the basis of pseudo-distance measurements originating from receivers disposed in ground stations and whose positions are very precisely known and close to the onboard receiver. The measurement of the pseudo-distance separating a ground receiver and a satellite is compared with the theoretical distance separating these two devices. The theoretical distance is calculated on the basis of the respective spatial coordinates of the ground receiver and of the satellite which are known at any instant. The difference between the distance measurement and the theoretical distance represents the measurement error; it is calculated for each satellite at each observation epoch. These distance differences constitute corrective terms (also called differential corrections) which are deducted from the pseudo-distance measurements carried out by the mobile receiver. These corrections have the effect of almost totally removing the errors which exhibit a significant spatial correlation whatever their origin, system or atmospheric. The corrections are all the more effective the closer the two receivers. However, the differential measurement does not eliminate the errors related to the reflections of the signal on objects that are close to the antenna of the receiver, nor the errors specific to the receiver (thermal noise). These errors are present in the reference receiver as well as in the onboard receiver, they degrade the positioning measurement during differential correction; the precision obtained is of the order of a few metres.
To improve the positioning precision, the ground receivers and carrier-borne mobile receivers can also exploit a second item of information formulated by the receiver which is the measurement of the phase of the carrier, for each satellite signal received. The measurement of the instantaneous phase of the carrier received actually makes it possible to calculate a pseudo-distance, termed the carrier pseudo-distance, between the receiver and the satellite, in the same way as the measurement of the instantaneous phase of the pseudo-random code. This carrier pseudo-distance undergoes the same variations as the code pseudo-distance, when the distance between the receiver and the satellite or the time bias due to the clock of the receiver vary. This pseudo-distance measured by the phase is a priori ambiguous since the phase is known modulo 2π but it is much less noisy than the code pseudo-distance measurements.
A known solution for improving pseudo-distance measurements consists in smoothing the noisy pseudo-distance measurement carried out on the code by the not very noisy phase measurements. For this purpose the receiver applies a low-pass filter to the difference between the code pseudo-distance and carrier pseudo-distance measurements, then adds this filtered difference to the carrier pseudo-distance measurement so as to reconstitute the code phase measurement. This processing is carried out satellite axis by satellite axis. If the measurement is differential, an identical smoothing is applied to the receivers of the ground station so that the tracking error of the low-pass filter, due to the divergence between the code and the carrier related to the fluctuations in the ionospheric delay, is identical on the ground and in the mobile receiver, and does not disturb the positioning measurement after application of the correction.
The main benefit of the axis-by-axis smoothing process lies in its simplicity and in the absence of any coupling effect between the measurements of the pseudo-distances of the various satellites or channels, nonetheless it is not completely satisfactory. Indeed, the gain in the precision of the measurement is important only when the smoothing is performed with a long time constant; and in this case, the duration of reinitialization to recover the precision after an abrupt modification of the set of available measurements is also long, for example upon the disappearance of a satellite by masking, upon a fault with a satellite or else upon a fault with a ground receiver in the case of differential GPS. It would be desirable to avoid this drawback.
Moreover, to quantify the integrity of the position measurement in applications where integrity is critical, such as aeronautical applications, a parameter called the position measurement “protection radius” is used. The protection radius corresponds to a maximum position error for a given probability of an error occurring. That is to say, the probability that the position error exceeds the stipulated protection radius without being warned thereof is less than this given probability value. The calculation is based on two types of error which are on the one hand normal measurement errors and on the other hand errors caused by an operating anomaly in the constellation of satellites, by a failure of a satellite or else a failure of a ground receiver.
Commonly, two undesirable types of events that may arise with a GPS positioning system may be distinguished: the first, dubbed non-continuity, corresponds to an unplanned but declared degradation in the service; the second, called non-integrity, corresponds to an erroneous but undeclared position measurement, whose appearance is that of a reliable measurement. A non-continuity can correspond for example to the occurrence of an alarm indicating the supposed presence of a failure. In both cases the consequences can be serious, hence the necessity to minimize the probability of such events. A protection radius calculation can be estimated for a non-continuity probability value and a non-integrity probability value that are given a priori. In this case the probability that the positioning error exceeds the radius will be less than the given non-integrity probability, and the probability of alarm (justified or unjustified) will be less than the given non-continuity probability.
For example, in the case of existing systems which are based on smoothing, axis by axis, the pseudo-random code position measurements by the phase variations of the carrier, the protection radius degrades abruptly as a satellite disappears and takes a long time to regain an acceptable value after the satellite reappears, because of the response time of the smoothing filter. It would be desirable to find a solution which avoids this drawback.
The system described in European patent EP 1 839 070 alleviates the drawbacks of the solutions presented hereinabove. Its aim is to improve the precision of the position measurement by using in a novel manner the two simultaneous measurements of pseudo-distances made available by each satellite while according the position thus obtained better robustness in relation to strong degradations of the geometry of the satellites (transient regime) that it is liable to encounter, doing so both for the absolute positioning and for the differential positioning.
Such a system implements scalar tracking, which consists in tracking the satellites independently of one another. In this conventional configuration, to synchronize the local code and the local carrier on the signal received from the satellite, the receiver uses two tracking devices in parallel: a code loop and a carrier loop.
The term tracking represents the capacity to synchronize with the satellite signals by using, through feedback, the phase discrepancies between the receiver signal and the satellite signals.
The code loop serves to position a local code in phase with the code contained in the signal received from a satellite, so as to carry out a correlation giving the maximum energy. The carrier loop serves to slave the frequency or the phase of the local carrier with that of the carrier received, so as also to maximize the result of the correlation.
The presence of a signal at the output of the correlation having a significant amplitude, i.e. markedly greater than would be given by the ambient noise in the absence of any satellite signal, signifies that the local code and the local carrier are synchronized with the signal received, thereby making it possible to measure at each instant the date of emission of the signal received, by way of the phase of the local code and of the phase of the local carrier.
The initial synchronization of the phase of the local code and of the local carrier on the signal received from the satellite is carried out in a preliminary so-called acquisition step. This step is not detailed here since it is outside the scope of the patent, and is known to the person skilled in the art. To summarize, it consists of an energy search by scanning a code-wise and Doppler-wise uncertainty domain until the correlation gives a sufficient signal level.
The code loop uses at least two correlation pathways, with a local code leading and a local code lagging with respect to the reference code of the punctual pathway, and the same carrier for the three pathways. The loop seeks to equalize the levels at the output of the two correlation pathways through feedback on the phase of the local code by virtue of:                a code discriminator which measures the energy difference between the two pathways,        a loop corrector which filters the output of the code discriminator and produces a speed correction, and        a code numerically-controlled oscillator or NCO which transforms the speed control into a local code phase.        
When the two levels are equal the reference code of the punctual pathway is in phase with the code of the satellite received, thereby ensuring maximum efficiency of the correlation on this pathway, except for the loss due to the Doppler effect.
There are two types of carrier loop: frequency loops and carrier phase loops.
A frequency loop serves to make the frequency of the local carrier coincide with that of the carrier received, firstly so as to maximize the signal gathered after correlation and secondly to provide a Doppler measurement on the tracked satellite. A phase loop serves to slave the phase of the local carrier to the phase of the carrier received (modulo 2π).
A phase loop provides much richer information than a frequency loop, since the evolutions of the measured carrier phase convey very precisely the evolutions of the pseudo-distances, without drift, in contradistinction to the measurements of an integrated frequency loop.
Scalar tracking is unlike vector tracking which, on the basis of the measurements made on each signal at the output of the correlation pathways, directly estimates the position and the speed of the receiver which then serve for closing the slaving loop by feeding the numerically-controlled oscillators the carrier phase and the code phase of each signal. Thus, the tracking of the satellites is done in a centralized manner at the level of the final navigation calculation affording better robustness to jamming. This allows the tracking of the signals at more significant interference levels, especially when the navigation calculation module is coupled with an inertial centre.
There are several types of vector trackings.
In “Fundamentals of Signal Tracking Theory” (Spilker), the tracking of the signals is done by extending the scalar code loop named DLL (Delay Lock loop) into a vector loop. The carrier loop is demodulated by virtue of a speed aid (“Code Only” tracking). Therefore, no benefit is derived from the information regarding the measurement of the carrier phase. In the conventional GPS navigation architecture coupled with an inertial centre, the phase is controlled from the navigation calculation module in the case of heavy jamming (“code only” configuration). It is the code loop which is independent and slaved satellite by satellite.
Other documents, such as “Comparison of Vector-Based Software receiver implementations with Application to Ultra-Tight GPS/INS Integration” (Petovello, Lachapelle), “Enhanced GPS receiver utilizing vector tracking, fast correlation processing and multiple signal characterization” (Heppe), and “Global positioning system and inertial measuring systems unit ultra-tight coupling” (Abbott), use the Doppler for the navigation calculation. The slaving of the carrier is then done by the Doppler (as opposed to the phase).